The present invention relates to transparent and colorable elastomeric compositions and, more particularly, to transparent and colorable elastomeric compositions that can be used in reinforcing applications having a high abrasion resistance and traction.
Rubber compositions are used in a variety of applications, including tire components such as treads and sidewalls, hoses, belts, footwear components, vibration isolation devices and bladders. While the particular rubber compositions used in each of these applications vary widely in their physical properties, one attribute remains the samexe2x80x94their color. Most rubber compositions are black. Furthermore, most rubber compositions eventually become discolored due to heat, light, ozone, etc. This is particularly true for rubbers used in stressful, demanding applications such as tire treads and sidewalls.
Practitioners in this field will point to the presence of the reinforcing filler xe2x80x9ccarbon blackxe2x80x9d as a prime reason that most rubbers are black. While this is true, carbon black is not the only factor. In fact, a wide variety of other fillers, curatives, antidegradants, oils and the rubbers themselves can all result in a dark color that is essentially impossible to pigment. This is evident in compositions where carbon black has been replaced with a silica filler and the rubber is still discolored. For example, European Patent 0 682 071 B1 discloses a silica reinforced tire tread which, due to the presence of the aromatic processing aid, coupling agent, antidegradants and a sulfur curative system, will still be dark in color. In fact, it is uncertain how many of the ingredients present in the rubber composition would have to be changed to produce a colorable composition.
There are some colorable and transparent elastomeric compositions that are currently used. For example, clear EPDM elastomers are available. However, these elastomers do not covulcanize with other rubbers. Since many rubber applications involve combining several types of rubber to form a single article (i.e. tires), these EPDM elastomers are limited in their usefulness. A related patent, serial number 09/592,757 (assigned to the assignee of the present invention), discloses improved blends of rubbers and other elastomers with silica to form transparent elastomeric materials. However, these materials may not be suitable for applications requiring high abrasion resistance while maintaining the colorability of the composition. Such is the case for, example, tire sidewalls and shoe soles.
White sidewalls on tires are a form of colorable rubber. The white color is achieved by using fillers such as silica, clay, talc and carbonates instead of carbon black and adding titanium dioxide as a whitening pigment. However, the white color comes with a price. The fillers are more fragile than carbon black and result in a weak rubber composition that does not reinforce the tire.
In applications such as shoe soles and tire treads where a large amount of a filler such as silica is used, it is desirable to maintain and adequately adjust certain physical properties such as the processability of the rubber, the cure rate, and final cure characteristics such as traction and abrasion resistance. In particular, abrasion resistance is important for colorable rubber compositions in, for example, tire sidewalls where there is a need for resistance to curb-scuffing. Also, in shoe soles and other shoe applications there is a need for an elastomeric composition that has long wear. An object of the present invention it to provide a colorable elastomeric composition that has improved abrasion resistance and other improved physical characteristics, while maintaining the colorability and transparency of the composition.
The present invention provides improved transparent and colorable elastomeric compositions. The transparent elastomeric compositions are halogenated isoolefin/para-alkylstyrene elastomers which can be alone in the composition or can be covulcanized with rubbers such as polybutadiene, polyisoprene, styrene-butadiene rubber, styrene-isoprene-butadiene rubber, isoprene-butadiene rubber, ethylene-propylene diene rubber, high cis-polybutadiene or natural rubber. The colorable rubber compositions have sufficient properties to function as a reinforcing member in an automobile tire or sufficient traction and abrasion resistance to function as a shoe sole. Preferably, both the transparent and colorable elastomeric compositions include at least one copolymer of a C4 to C7 isoolefin and a para-alkylstyrene, silica, a coupling agent, and a processing aid such as a low molecular weight saturated polymer.
The xe2x80x9celastomeric compositionxe2x80x9d of the present invention is a mixture of at least the halogenated isoolefin/para-alkylstyrene, a filler agent (described below), and the processing aid (described below). The elastomeric composition may also contain other rubbers as listed above, as well as a cure system when another rubber is present.
The elastomeric compositions of the present invention are useful in a variety of applications, particularly pneumatic tire components, hoses, belts, solid tires, footwear components, rollers for graphic arts applications, vibration isolation devices, pharmaceutical devices, adhesives, sealants, protective coatings and bladders for fluid retention and curing purposes.
The present invention is an elastomeric composition comprising a copolymer of a C4 to C7 isoolefin and a para-alkylstyrene, a filler; and a low molecular weight, saturated processing aid. The composition may also comprise a rubber selected from the group consisting of polybutadiene, polyisoprene, styrene-butadiene rubber, styrene-isoprene-butadiene rubber, Isoprene-butadiene rubber, ethylene-propylene diene rubber, high cis-polybutadiene or blends thereof.
The colorable elastomeric compositions of the present invention have sufficient properties to function as shoe soles and other shoe components and as a reinforcing member in an automobile tire, as well as other applications where a colorable, transparent material is desired. The colorable elastomeric compositions of the present invention are useful in making colored elastomeric products capable of meeting current performance requirements. These colorable compounds were produced by replacing carbon black filler with a non-staining mineral filler such as, but not limited to, fumed or precipitated silicas, clays, talcs, calcium carbonates, aluminum oxides, titanium oxides, and zinc oxides. The mineral filler must reinforce the polymer system and not inhibit pigmentation to be effective. In addition, the remaining components of the colorable compound were selected on the basis that they will not interfere with the colorable nature of the elastomer. The cured, colorable compounds of the present invention still have the same dynamic and physical properties that meet the performance demands of current black-colored tire treads.
All components of the transparent and colorable elastomeric compositions must be carefully selected so that they will not interfere with the transparency and/or colorability of the composition. For example, the elastomers, fillers, processing aids, antidegradants and curatives should not discolor the composition during the formation of the elastomeric composition. Furthermore, the components should not discolor the elastomeric composition as a result of exposure to light (including UV), heat, oxygen, ozone and strain.
In one embodiment of the present invention, an elastomeric composition is produced which exhibits transparent properties. The term xe2x80x9ctransparentxe2x80x9d, as used herein is defined as transmission of light without substantial scattering such that visual identification can be made of objects behind the elastomeric composition. Degrees of transparency can vary from contact transparency to complete transparency. However, other embodiments of the invention are not limited to transparent compositions, such as those blended for tire treads.
Isoolefin and Para-alkylstyrene Copolymer
Preferably, the elastomeric composition contains at least one copolymer of a C4 to C7 isoolefin and a para-alkylstyrene. Preferably, the C4 to C7 isoolefin is isobutylene. In addition, the para-alkylstyrene is preferably para-methylstyrene. Most preferably, the copolymer is a terpolymer of isobutylene, para-methylstyrene and bromo para-methylstyrene. The copolymer used in the transparent elastomeric compositions of the present invention is preferably a terpolymer of isobutylene, para-methylstyrene and bromo para-methylstyrene (BrPMS). In addition, this terpolymer preferably composes from 10 to 100 phr of the transparent elastomeric composition. More preferably, the terpolymer composes from 30 to 80 phr of the transparent elastomeric composition. More preferably, the terpolymer composes from 20 to 50 phr of the composition. Preferred commercial examples of such terpolymers are EXXPRO(trademark) Elastomers (ExxonMobil Chemical Company).
The copolymer of a C4 to C7 isoolefin and a para-alkylstyrene of the present invention also encompasses terpolymers of a C4 to C7 isoolefin, para-alkylstyrene and halogenated para-alkylstyrene. The percentages of para-alkylstyrene and halogenation can vary widely. Different applications may require dramatically different formulations. Generally, the copolymer of the present invention will have from 2 wt. % to 20 wt. % para-alkylstyrene (preferably para-methylstyrene). In addition, the copolymer of the present invention will have from 0.20 mol % to 3.0 mol % of a halogenated compound, such as bromo para-methylstyrene.
Preferably, low levels of either bromine and/or para-alkylstyrene are used. In a preferred embodiment, para-alkylstyrene (preferably para-methylstyrene) comprises from 5 wt. % to 15 wt. % of the copolymer. More preferably, it is from 5 wt. % to 7.5 wt. % of the copolymer. In another preferred embodiment, a halogenated compound, such as bromo para-methylstyrene comprises from 0.20 mol % to 3.0 mol % of the copolymer. More preferably, it comprises from 0.50 mol % to 1.5 mol % of the copolymer. Most preferably, it is from 0.5 mol % to 1.0 mol % of the copolymer.
Filler
The elastomeric composition also contains a filler. The transparent elastomer compositions of the present invention do not contain carbon black. The transparent feature of the composition is obtained in part by using fillers to composing from 10 to 100 parts, per hundred parts of rubber (phr), of the composition which are finer than the wavelength of visible light. Silica is preferred as the filler, however other non-black fillers such as clays, talcs and other mineral fillers may be used. Silica may also be used to such an extent that the composition is no longer transparent or colorable.
The colorable compositions of the present invention are produced by replacing carbon black filler with a non-staining mineral filler such as, but not limited to, fumed or precipitated silicas, clays, talcs, calcium carbonates, aluminum oxides, titanium oxides, and zinc oxides. The preferred filler is silica present in the composition from 10 to 100 phr. The silica used in the transparent elastomeric compositions of the present invention is preferably a mixture of fumed and precipitated silicas. Also, the precipitated silica preferably composes from 30 to 80 parts of the transparent elastomeric composition. More preferably, it composes from 40 to 70 parts. The coupling agent used in the transparent elastomeric compositions of the present invention is preferably an organosilane-coupling agent. Preferably, the organosilane-coupling agent composes from 2 to 15 weight percent, based on the weight of silica, of the transparent elastomeric composition. More preferably, it composes from 4 to 12 weight percent of the composition.
The fillers of the present invention may be any size and typically range, e.g., in the tire industry, from about 0.0001 to about 100 microns. As used herein, silica is meant to refer to any type or particle size silica or another silicic acid derivative, or silicic acid, processed by solution, pyrogenic or the like methods and having a surface area, including untreated, precipitated silica, crystalline silica, colloidal silica, aluminum or calcium silicates, fumed silica, and the like.
One or more coupling agents are preferably used in the elastomeric compositions of the present invention. More preferably, the coupling agent is a bifunctional organosilane cross-linking agent. By an xe2x80x9corganosilane cross-linking agentxe2x80x9d is meant any silane coupled filler and/or cross linking activator and/or silane reinforcing agent known to those skilled in the art including, but not limited to, vinyl triethoxysilane, vinyl-tris-(beta-methoxyethoxy)silane, methacryloylpropyltrimethoxysilane, gamma-amino-propyl triethoxysilane (sold commercially as A1100 by Witco), gamma-mercaptopropyltrimethoxysilane (A189 by Witco) and the like, and mixtures thereof. In a preferred embodiment, bis-(3(triethoxysilyl)-propyl)-tetrasulfane (sold commercially as Si69 by Degussa) is employed.
Processing Aid
A processing aid is also present in the composition of the invention. The aid is present from 2-30 phr, more preferably from 5-20 phr, and most preferably from 10-20 phr. A typical processing aid is one that will enhance the transparent or colorable nature of the elastomeric composition. Some commercial examples of processing aids are SUNDEX(trademark) (Sun Chemicals) and FLEXON(trademark) (ExxonMobil Chemical). Preferably, the processing aid does not contain aromatic or unsaturation. Processing aids include, but are not limited to, plasticizers, tackifiers, extenders, chemical conditioners, homogenizing agents and peptizers such as mercaptans, petroleum and vulcanized vegetable oils, waxes, resins, rosins, and the like. The preferred processing aid is a low molecular weight, saturated polymer such as polybutene. Commercial examples of such a processing aid are the PARAPOL(trademark) Series of processing aids, such as PARAPOL(trademark) 950 and PARAPOL(trademark) 2500, both from ExxonMobil Chemical Company (Also sold under the name INFINEUM(trademark) C9925 and INFINEUM(trademark) C9995 by Infineum International Limited).
More specifically, the PARAPOL(trademark) Series processing aids are polymers of isobutylene and butene, each individual formulation having a small range of molecular weights for each formulation, all of which can be used in the composition of the invention. The molecular weights of the PARAPOL(trademark) processing aids are from 420 MN (PARAPOL(trademark) 450) to 2700 MN (PARAPOL(trademark) 2500). The viscosity of, for example, of PARAPOL(trademark) 950 is 230 cSt at 100xc2x0 C., while the viscosity of PARAPOL(trademark) 2500 is 4400 cSt at 100xc2x0 C. The density (g/mL) of PARAPOL(trademark) processing aids varies from about 0.85 (PARAPOL(trademark) 450) to 0.91 (PARAPOL(trademark) 2500). The bromine number (CG/G) for PARAPOL(trademark) processing aids ranges from 40 for the 450 MN processing aid, to 8 for the 2700 MN processing aid. In the composition of the invention, the amount of the processing aid can be varied as well as the molecular weight (and hence, level of viscosity) of the processing aid. Thus, PARAPOL(trademark) 450 can be used when low viscosity is desired in the composition, while PARAPOL(trademark) 2500 can be used when a higher viscosity is desired. In this manner, the physical properties of the elastomeric composition can be controlled.
Additional Rubber Component
Another rubber component may also be present in the elastomeric composition of the invention. The rubber may compose from 0 to 90 phr, preferably from 20 to 80 phr. The transparent elastomeric compositions of the present invention are halogenated isoolefin/para-alkylstyrene terpolymers that can be covulcanized with polybutadiene, polyisoprene, styrene-butadiene rubber, styrene-isoprene-butadiene rubber, isoprene-butadiene rubber, ethylene-propylene diene rubber, high cis-polybutadiene or natural rubber. Some commercial examples of rubbers are NATSYN(trademark) (Goodyear Chemical Company), natural rubber (SMR 20), and BUDENE(trademark) 1207 or BR 1207 (Goodyear Chemical Company). The preferable covulcanate is high cis-polybutadiene (BR). By xe2x80x9ccis-polybutadienexe2x80x9d or xe2x80x9chigh cis-polybutadienexe2x80x9d, it is meant that 1, 4-cis polybutadiene is used, wherein the amount of cis component is at least 95%. An example of high cis-polybutadiene commercial products used in the covulcanized composition BR 1207.
In a preferred embodiment, the transparent elastomeric compositions of the present invention contains from 10 to 100 phr, of the copolymer of a C4 to C7 isoolefin and a para-alkylstyrene; from 10 to 100 phr of silica; from 0 to 20 weight percent based on the weight of the silica of a coupling agent; and 2-30 phr of a processing aid (discussed further below). Preferably, the colorable or transparent elastomeric compositions will also contain from 10 to 90 phr of polybutadiene, polyisoprene, styrene-butadiene rubber, styrene-isoprene-butadiene rubber, isoprene-butadiene rubber, ethylene-propylene diene rubber, high cis-polybutadiene or blends thereof. More preferably, the transparent elastomeric compositions will contain from 20 to 80 phr of polybutadiene, polyisoprene, styrene-butadiene rubber, styrene-isoprene-butadiene rubber, isoprene-butadiene rubber, ethylene-propylene diene rubber, high cis-polybutadiene or blends thereof.
Crosslinkers and Accelerators
The compositions produced in accordance with the present invention typically contain other components and additives customarily used in rubber mixes, such as effective amounts of other nondiscolored and nondiscoloring processing aids, pigments, accelerators, cross-linking and curing materials, antioxidants, antiozonants, fillers and naphthenic, aromatic or paraffinic extender oils if the presence of an extension oil is desired. Accelerators include amines, guanidines, thioureas, thiazoles, thiurams, sulfenamides, sulfenimides, thiocarbamates, xanthates, and the like. Cross-linking and curing agents include sulfur, zinc oxide, and fatty acids. Peroxide cure systems may also be used.
Generally, polymer blends, e.g., those used to produce tires, are crosslinked. It is known that the physical properties, performance characteristics, and durability of vulcanized rubber compounds are directly related to the number (crosslink density) and type of crosslinks formed during the vulcanization reaction. (See, e.g., The Post Vulcanization Stabilization for NR, W. F. Helt, B. H. To and W. W. Paris, Rubber World, August 1991, pp. 18-23 which is incorporated by reference herein.) Generally, polymer blends may be crosslinked by adding curative molecules, for example sulfur, metal oxides (i.e., zinc oxide), organometallic compounds, radical initiators, etc. followed by heating. In particular, the following are common curatives that will function in the present invention: ZnO, CaO, MgO, Al2O3, CrO3, FeO, Fe2O3, and NiO. These metal oxides can be used in conjunction with the corresponding metal stearate complex and either a sulfur compound or an alkylperoxide compound. (See also, Formulation Design and Curing Characteristics of NBR Mixes for Seals, Rubber World, September 1993, pp. 25-30 which is incorporated by reference herein). This method may be accelerated and is often used for the vulcanization of elastomer blends.
The acceleration of the cure process is accomplished by adding to the composition an amount of an accelerant, often an organic compound. The mechanism for accelerated vulcanization of natural rubber involves complex interactions between the curative, accelerator, activators and polymers. Ideally, all of the available curative is consumed in the formation of effective crosslinks which join together two polymer chains and enhance the overall strength of the polymer matrix. Numerous accelerators are known in the art and include, but are not limited to, the following: stearic acid, diphenyl guanidine (DPG), tetramethylthiuram disulfide (TMTD), 4,4xe2x80x2-dithiodimorpholine (DTDM), tetrabutylthiuram disulfide (TBTD), benzothiazyl disulfide (MBTS), hexamethylene-1,6-bisthiosulfate disodium salt dihydrate (sold commercially as DURALINK HTS by Flexsys), 2-(morpholinothio) benzothiazole (MBS or MOR), blends of 90% MOR and 10% MBTS (MOR 90), and N-oxydiethylene thiocarbamyl-N-oxydiethylene sulfonamide (OTOS) zinc 2-ethyl hexanoate (ZEH), N, Nxe2x80x2-diethyl thiourea (thiourea) (sold commercially as Thiate U by R. T. Vanderbilt).
The present invention provides improved elastomeric compositions comprising a copolymer of a C4 to C7 isoolefin and a para-alkylstyrene, silica, a processing aid, and optionally, one or more coupling agents. In order to improve certain physical properties of the composition, another rubber is also present. These compositions exhibit improved properties including improved abrasion resistance, reduced cut growth, improved adhesion, reduced heat build-up, and retention of mechanical properties during severe heat build-up conditions such as those experienced in xe2x80x9crun-flatxe2x80x9d tires and engine mounts for transportation vehicles. The substantially isoolefin (isobutylene) backbone elastomer is a key element in that it imparts a self-limiting heat build-up. At lower temperatures, these elastomers exhibit high damping behavior which dissipates mechanical energy in the form of heat. However, as the elastomer heats up, the damping behavior diminishes and the behavior of the elastomer in more elastic and less dissipative.
The materials are mixed by conventional means known to those skilled in the art, in a single step or in stages. For example, the elastomers of this invention can be processed in one step. In a preferred embodiment, the silica and silane are added in a different stage from zinc oxide and other cure activators and accelerators. In a more preferred embodiment, antioxidants, antiozonants and processing materials are added in a stage after silica and silane have been processed with the rubber, and zinc oxide is added at a final stage to maximize compound modulus. Thus, a two to three (or more) stage processing sequence is preferred. Additional stages may involve incremental additions of filler and processing aids.
The elastomeric compositions of the present invention are not only capable of being transparent or colorable, but can be covulcanized with other rubbers. The transparency will depend upon the amount of filler used. This results in an elastomer that can be used in wide variety of applications outside of the uses for known elastomers. The elastomeric compositions of the present invention are useful in a variety of applications, particularly pneumatic tire components, hoses, belts, solid tires, footwear components, rollers for graphic arts applications, vibration isolation devices, pharmaceutical devices, adhesives, sealants, protective coatings and bladders for fluid retention and curing purposes. In particular, the elastomeric compositions of the present invention can be used in shoe soles and tires.
The colorable elastomeric compositions of the present invention exhibit improved hysteretic properties, traction, heat stability and retention of properties upon aging to known colorable elastomers. This results in colorable rubber compositions which have sufficient properties to function as a reinforcing member in an automobile tire. The colorable rubber will allow a manufacturer to produce a tire with improved product appearance.
Below are examples of various compositions and methods of forming the composition of the invention. The following examples are by no means meant to be limiting of the invention, but are representative only. Cure properties were measured using a MDR 2000 at the indicated temperature and 0.5 degree arc. Test specimens were cured at the indicated temperature, typically from 150xc2x0 C. to 160xc2x0 C., for a time corresponding to T90+ appropriate mold lag. When possible, standard ASTM tests were used to determine the cured compound physical properties. Stress/strain properties (tensile strength, elongation at break, modulus values, energy to break) were measured at room temperature using an Instron 4202. Shore A hardness was measured at room temperature by using a Zwick Duromatic. Abrasion loss was determined at room temperature by weight difference by using an APH-40 Abrasion Tester with rotating sample holder (5 N counter balance) and rotating drum. Weight losses were indexed to that of the standard DIN compound with lower losses indicative of a higher abrasion resistance index.
Dynamic properties (G*, Gxe2x80x2, Gxe2x80x3 and tangent delta) were determined using a MTS 831 mechanical spectrometer for pure shear specimens (double lap shear geometry) at temperatures of xe2x88x9220xc2x0 C., 0xc2x0 C. and 60xc2x0 C. using a 1 Hz frequency at 0. 1, 2 and 10% strains. Temperature-dependent (xe2x88x9280xc2x0 C. to 60xc2x0 C.) dynamic properties were obtained using a Rheometrics ARES at Sid Richardson Carbon Company, Fort Worth, Tex. and at ExxonMobil Chemical, Baytown, Tex. A rectangular torsion sample geometry was tested at 1 Hz and appropriate strain. Values of Gxe2x80x3 or tangent delta measured at 0xc2x0 C. in laboratory dynamic testing can be used as predictors of tire traction for carbon black-filled BR/sSBR (styrene-butadiene rubber) compounds. Temperature-dependent (xe2x88x9290xc2x0 C. to 60xc2x0 C.) high-frequency acoustic measurements were performed at Sid Richardson Carbon Company using a frequency of 1 MHz and ethanol as the fluid medium.